Conventionally, there have been known power modules in which a power chip including a semiconductor device such as Insulated Gate Bipolar Transistor (IGBT) is mounted on a leadframe therein, and of which the whole system thereof is molded with resin. Since such a semiconductor device produces heat during an operating state, it is common to dispose a heat sink via an insulating layer on a back side surface of the leadframe in order to cool the semiconductor device.